Analysis: Pivotal congressional races six months ahead of election

Six months from Tuesday, Americans will reset the power equation in Washington, and not only for the presidency. Control of Congress is also at stake.

In each of the last three national elections, voters opted for change, and they may be about to do so again. In 2006, they turned the House of Representatives and the Senate from Republican to Democrat. In 2008, they did the same at the White House. Then, in 2010, they flipped the House back to the Republicans.

Amid a sluggish economic recovery that has yet to reach millions of Americans, the Democratic president is now a vulnerable incumbent and his party’s control of the Senate is in jeopardy. The House, at least for now, seems less likely to flip again.

In part, the Democrats are victims of their success. Senators serve for six years, and largely as a result of their 2006 sweep, 23 of the 33 seats in this fall's election are in Democratic hands (or, in the case of retiring Sen. Joe Lieberman of Connecticut and Sen. Bernie Sanders of Vermont, independents who vote with Democrats in choosing the majority leader).

The current Democratic edge in the Senate is six seats, down sharply from the 20 seat-majority during the first year of President Obama's term. Lists of competitive Senate races show more than twice as many Democratic seats at risk.

It’s not hard to see Republicans netting the three seats they need to assume power, with larger gains within the range of possibility. The GOP needs four to take control if Obama is reelected, because Vice President Joe Biden would break a 50-50 tie in the chamber, as then-Vice President Dick Cheney did in 2001. (More detail on individual Senate contests below).

The House picture is clearer, though Republican Speaker John Boehner caused a stir not long ago when he told Fox News there was "a one-in-three chance" his party could lose the House. That comment may have had more to do with prodding reluctant Republican donors, and guarding against complacency, than with any big shift in the November outlook.

At the moment, a modest Democratic pickup of fewer than 10 House seats seems the likeliest outcome -- well short of the 25 needed to knock the Republicans from power. That's essentially the conclusion of Charlie Cook and Stu Rothenberg, respected nonpartisan analysts and friendly competitors who actually look at congressional elections on a race-by-race basis (and whose handicapping forms the basis for much of this report.)

The national election in November will be the first since House districts were redrawn after the 2010 census. But in spite of fairly marked shifts within some states, including California, Illinois, North Carolina and Georgia, reapportionment turned out to be a wash, overall.

Nationwide, a dozen House seats are forecast to change party, through retirements, scandal or redistricting, with Republicans expected to net a total of just two as a result. An additional two dozen districts, equally divided between the parties, will have tossup elections. A more expansive definition of competitive House races puts between 50 and 70 seats in play, with more Republican than Democratic seats at risk.

Could a national wave sweep dozens of incumbents from office? Those tsunamis are hard to predict and sometimes develop late, but there are few indications that one is building. Far more likely is that 85% or more of the nation's congressional districts will stay in their current party alignment (and upward of 95% of incumbents who seek reelection will be returned to office.) Independent analysts foresee a familiar story: The powers of incumbency, a partisan redistricting process and loosened limits on campaign spending will combine to restrict competition at the polls and, as a consequence, increase the odds that polarization and stalemate will continue in Washington.

There are competitive Senate races in perhaps a dozen states, roughly equal to the number of states in which the presidency will be decided. At least five states hit the jackpot -- as both presidential and Senate battlegrounds, guaranteeing an almost inconceivable deluge of campaign commercials throughout the summer and fall. They are: Florida, Ohio, Virginia, Nevada and Wisconsin.

A quartet of genuine tossup races, for seats currently divided between Democrats and Republicans, could go down to the wire:

In Virginia, Democrat Tim Kaine and Republican George Allen, both former governors, are vying to replace retiring one-term Democrat Jim Webb.

Montana’s freshman Democrat, Jon Tester, is defending his vulnerable seat against popular Republican Denny Rehberg, who represents the entire state in the House.

In Massachusetts, moderate first-term Republican Scott Brown, who replaced the late Democratic icon Edward M. Kennedy, is being challenged by liberal Harvard law professor Elizabeth Warren.